Is this how the Infiniti EX35 is designed or is it just my car that has this problem?

Thank you.

I think for heat, they force you to use outside air as recirculating inside air might make the air too dry.

On the other hand, have you ever had a car (once up to temperature) blow cold air when the heat was on? I haven't. You can always get blazing hot air even when taking in outside air.

They probably allow recirculating of air for A/C because A/C has to work a lot harder to cool air vs heating air where it simply uses engine heat (which is abundant).

- KJ

I believe the main reason they do this is if you have re-circulated air on (especially if it's cool or damp outside) then the inside air that you're breathing in and out gets damp, so without a/c on (which removes moisture from air) the air inside the car would get more and more moist, and would steam up the windows.

Also if you press the front defogger button it automatically turns the a/c on to ensure it's blowing only dry air onto the front screen.

Yup, agreed, it's a safety thing...prevents your windows from getting fogged up. The AC condenser removes that moisture and puts it OUTSIDE of your EX. Other car brands do this as well.